Methylnitrene

Singlet methylnitrene is converted into methyleneimine in a very exothermic process. Analysis of the vibrational structure of the PES spectrum indicates that the species detected belongs to a resonance rather than a true minimum on the singlet MeN potential surface. ... 

These conclusions were anticipated by product studies. Alkyl azides are readily available and their thermal and photochemical decomposition reactions have been studied. In general, light and heat induced decomposition of methyl azide does not produce a MeN species that can be intercepted in respectable yields with a bimolecular trap. For example, attempts to trap MeN with cyclohexane as solvent produced only a 0.4% yield of adduct. Photolysis of CH3N3 or CD3N3 at cryogenic temperatures fails to produce an IR spectrum attributable to triplet methylnitrene. The IR spectrum of CH2=NH (or CD2=ND) is observed instead. " ... 

Triplet methylnitrene has been produced in the gas phase in a corona discharge. This process has allowed its highly resolved absorption spectrum to be recorded and analyzed. 

NO on the basis of NMR and IR spectroscopy. At low temperatures, an unstable complex with /no at 1670 cm S assigned to the bent nitrosyl of (39), was observed. By monitoring the NMR spectrum at room temperature, researchers observed a signal they assigned to the N-methyl group of (40), and rj -nitrosomethane complexes have been isolated and structurally characterized (21 i). This complex then decomposes, either by a binuclear mechanism or by unimolecular elimination of methylnitrene, to give azomethane and the resulting oxocomplex (41). 

Since no ethane could be detected, it was felt that methane had to be formed directly, and not by way of methyl radicals. It was proposed that methane arose from two methylnitrene molecules reacting at a surface to give methane and diazomethane. There was, however,... 

The majority of recent research in the photochemistry of azides and the generation of nitrenes has involved aryl rather than alkyl azides. Amongst the latter, however, is an interesting study of the photodissociation of methyl azide at wavelengths from 292 to 325 nm. The photodissociation dynamics of methyl azide are apparently complex, with the predominant pathway producing CH2=NH by concerted 1,2-H shift and N2 extrusion. Triplet methylnitrene (CH3N) was also observed by emission spectroscopy, but seems to arise by a minor, spin-forbidden pathway. 

Aminocarbene (NHa—CH) is probably more stable than methylnitrene ( — CHs) due to stabilization by the amine lone pair H2N—CHg—CH... 

Easy thermal cleavage of the N-N bond in pyridine N-methylimines66 has precluded isolation of such compounds. The other cleavage product, presumably methylnitrene, has not been detected or trapped, but phenyl-nitrene was detected in the thermal cleavage of the N-N bond of 1,2,3-triazole N-phenylimines.106... 

Methylnitrene is the simplest of all alkylnitrenes. Photolysis of methyl azide (CH3N3, 13) does not produce 1NCH3 ( 14) as a trappable species, and even attempts to detect it by femtosecond flash photolysis have failed.Triplet methylnitrene was not formed on direct irradiation of 13, and only methyleneimine (15), the product of formal isomerization of 14, was detected in cryogenic matrices. 

Triplet methylnitrene can be produced by sensitized photolysis in the condensed phase or in a gas-phase corona discharge. " Barash et al. and Wasserman reported a value of the zero-field splitting parameter D/hc of 1.595 cm" for matrix isolated triplet CHgN. This earlier experiment is questionable, and a revised value ( D/hc = 1.720 cm" ) was obtained for CH3N using gas-phase spectroscopy. The triplet-triplet absorption spectrum " of 14 and its emission spectrum " " are well known. The 0-0 transition in the absorption spectrum of 14 was found to occur at 316.9 nm in an Ng matrix and at 314.3 nm in the gas phase, similar to the spectrum of the parent NH (336 nm). ... 

Thus, in a period of only a few years (1980 to 1987), methylnitrene in its triplet ground state had gone from the tennons category to what can be considered the best characterized of all known organic nitrenes. Nevertheless, the question of whether singlet methylnitrene 14 is an intermediate in the thermal and photochemical decomposition of 13 remained nnsolved. 

A very shallow minimum for singlet methylnitrene in the A state was found, and its rearrangement to 15 was studied using the CAS(10,8)/MP2 procedure. It was possible to localize a transition state of Cj symmetry only 1.4 kcal/mol above the minimum of singlet nitrene. 

The CASSCF(12,ll)/cc-pVTZ optimized geometries for the lowest singlet state of methylnitrene, methyleneimine, md the transition state that connects these two energy minima are shown in Figure 5.6. The CASSCF(12,11) and CASPT2 energies are given in Table 5.2. 

Figure 5.6 CASSCF(12,ll)/cc-pVTZ geometries (bond lengths in angstroms, bond angles in degrees) of the stationary points along the lowest-energy pathway for the rearrangement of the lowest singlet state of methylnitrene (W MN) to methyleneimine (MI). ...

Table 5.2 Relative CASSCF(12,11) Zero-Point Energies (ZPEs) and CASSCF(12,11) and CASPT2(12,11) ZPE-Corrected Electronic Energies (kcaPmol) of 14 and Methylnitrene 14 and Methyleneimine 15 and of the Transition Structures Connecting A 14 to 15 (TSl) Computed with cc-pVDZ and cc-pVTZ Basis Sets ...

It is in the (A state that migration of the imique hydrogen can most easily occur. In fact, the lowest-frequency vibrational mode of 564 cm in this state of methylnitrene corresponds to an a motion that moves this hydrogen toward the nitrogen. 

Based on the results of these CASSCF/CASPT2 calculations, singlet methylnitrene is indeed predicted to be an energy minimum rather than a transition state the barrier to rearrangement is most likely in the range of 2.5 1 kcal/mol. Thus, at least in principle, 14 should be an observ-... 

Recently, Tsao et al. performed CASSCF/CASPT2 calculations of the properties of the cyclopropylnitrene (16). This study was motivated by the fact that cyclopropylcarbene can be chemically intercepted and has a lifetime of about 20 ns in solution at ambient temperature. Recall that singlet methylcarbene and singlet methylnitrene have not been detected or chemically intercepted and are predicted to isomerize to, respectively, ethylene and imine over a very small barrier of 1 to 3 kcal/mol. Calculations were performed to estimate the lifetime of 16 and its suitability for laser flash photolysis studies. ... 

The CASPT2(8,8)/6-31G(2d,p) calculations on structures along the intrinsic reaction coordinate (IRC) path at the HF/6-31G level predict that the fragmentation reaction is barrierless, but the ring-expansion reaction has a small activation barrier (=2.4 kcal/mol). Therefore, the calculations indicate that, unlike cyclopropylcarbene, singlet cyclopropylnitrene will have a very short lifetime in solution at ambient temperature, and, like methylcarbene and methylnitrene, 16 will be very difficult to detect. ... 

If a free singlet nitrene is an intermediate, produced upon decomposition of a vinylazide, then the activation energy of vinyl nitrene rearrangement to azirine must be very small. The situation is similar to the case of methylnitrene, produced upon photolysis of methyl azide (see Section 5.3). 

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